It is known that in the case of many oil shales and tar sands, the viscosity of the high molecular weight petroleum material may be reduced substantially by mixing such materials with a light petroleum base solvent, such as naphtha. In almost all cases, however, there remains a substantial proportion of material referred to as kerogens, which resist solubilization and which remain inextricably associated with the substrate solids causing the proposed recovery processes, based upon the use of petroleum base solvents, to be only partially effective and economically impractical.